1. Field of the Invention
This invention relates to apparatus for controlling the flow of power and dynamic voltage regulation on ac transmission lines, and more particularly, to economical apparatus for providing reactive and positive real power compensation in such ac transmission lines.
2. Background of Information
Traditionally, the only device available to power system engineers for controlling the direction and magnitude of power flow in a transmission line was the Phase-Angle Regulator (PAR). PARs typically consist of inter-connected transformers, one of which is essentially a large load tap changer.
Despite the flexibility that PARs introduce to the system operator, they never acquired widespread usage because they are slow and they have a relatively poor reputation for reliability. Also, they introduce a high series impedance in the line. The slow speed and poor reputation for reliability are mainly attributable to the use of mechanical load tap changers. Recent improvements in load tap changers seem to have improved reliability perceptions, but the slow speed (several seconds per tap change and a minute or more for major angle changes) renders these devices unusable for all but steady-state concerns. The high series impedance results from the connection of the two transformers. The leakage reactance of the series winding always appears in series with the supply transmission line whenever the PAR is in service. The leakage reactance of the excitation winding also appears in series with the supply transmission line, but varies with the angle shift.
The reactance inserted in a line by a phase shifter can be quite significant. Indeed, it is not uncommon, particularly on cable circuits, for the reactance of the PAR to exceed the reactance of the line with which it is placed in series. An additional consequence of the relatively high impedance of PARs is that they can consume significant amounts of reactive power at high power transfer levels; normally, a large reactive power source must be located close to a PAR to ensure adequate voltage regulation during contingencies.
Because of its slow control and relatively high impedance, the existence of a PAR on the system is normally considered detrimental during incidents where transient or dynamic stability may be a concern. Even though there has been some consideration recently to increasing the switching speed (by using thyristor switching) for transient and dynamic stability situations, the inherent high reactance of PARs can be a significant problem. Considering these problems, it is no surprise that Phase-Angle Regulators have been infrequently used on transmission systems, even though they provide the system operator with a degree of control of transmission line flows that could not be matched by any other existing device.
Recently, the concept of using a three-phase inverter to insert a voltage into a transmission line via a series transformer to facilitate transmission of power has been proposed. In essence, the exciting transformer is replaced by a three-phase inverter. The inverter, employing Gate Turn-Off (GTO) thyristors, is capable of either supplying vars to the transmission line or absorbing vars from the transmission line as a free-standing device. When the inverter is used in this fashion, the device is called a Series Power-Flow Controller (SPFC). Such a controller is disclosed in U.S. Pat. No. No. 5,198,746.
The SPFC injects a voltage in quadrature with the line current. Thus, the SPFC can only provide reactive power compensation. It has no capability to absorb or generate real power. Although it is convenient to think of the SPFC as analogous to a variable series capacitor or reactor, it is considerably more powerful than this. However, a short coming that the SPFC shares with variable series capacitors is that it has little capability to transfer real power at very low system angles because of its limited capability to introduce a phase shift at low system angles.
To introduce a phase shift, an additional dimension is required. Specifically, real power must be injected by the series inverter. To the extent that the real and reactive power insertions differ from real and reactive power consumption of the line, the difference shows up as an injected phase angle. If the real power interchange can be effected in both directions (i.e., from the device to the system and vice-versa), the resulting device is called a Unified Power Flow Controller (UPFC). Such a device is described in U.S. Pat. No. 5,343,139. This device has the capability of controlling reactive power flow, transmission line impedance, transmission line voltage magnitude, and transmission line voltage phase angle. The real power is drawn from the transmission line by an ac to dc converter and supplied to the series inverter through a dc link. The UPFC provides a great deal of flexibility in power flow control; however, the shunt ac to dc converter adds significantly to the cost and complexity of the apparatus.
Commonly owned U.S. patent application Ser. No. 08/368,947 filed on Jan. 5, 1995 (now U.S. Pat. No. 5,469,044), suggests that for applications in which the control requirements are asymmetrical, the MVA ratings of the series inverter and shunt converter can be reduced by vectorially adding a bias voltage to the voltage generated by the series inverter. However, this arrangement still requires the series inverter and a shunt connected AC to DC converter. Commonly owned U.S. patent application Ser. No.08/366,646 filed on Dec. 30, 1994 discloses a series-connected inverter which can supply both positive and negative real power in addition to reactive power compensation to control oscillations on a transmission line. The real power requirements are provided by devices which are independent of the transmission line. Both positive and negative real power can be provided by an energy storage device such as a battery or a super-conducting magnet. Where less dynamic compensation is required to control oscillations, only positive real impedance in the form of a resistive impedance can be used to absorb real power from the transmission line during positive real power surges. These devices require switching control to regulate the flow of real power.
U.S. Pat. No. 5,329,222 discloses a dynamic voltage restorer in which a series inverter injects a voltage into the transmission line to compensate for voltage sags. The real power needed to generate this injected voltage is provided by an energy storage device such as a capacitor, and preferably a converter such as a chopper is utilized between the capacitor and the converter to provide a constant dc input to the series inverter.
There are applications where only positive real power needs to be injected for controlling power flow or for dynamic voltage regulation.
There is a need, therefore, for apparatus for controlling power flow on an alternating current transmission system which does not require a shunt converter or energy storage devices with the accompanying switching apparatus in order to provide real power compensation.
There is a related need for an improved apparatus of this type which is economical.